1. Field Of The Invention
The present invention relates to viscosified fluids for stimulating subterranean formations or diverting in subterranean formations and to a method for delivering a material into a subterranean formation and more particularly a method for breaking a viscosified fluid in a subterranean formation.
2. Prior Art
In the drilling of a well and the subsequent recovery of fluids such as crude oil and natural gas from subterranean formations, various materials are added to the well bore or subterranean formation to improve the efficiency of the well drilling operation, to increase production of fluids from the formation and in some instances to terminate or seal portions of non-producing zones or wells. The treating agents are generally added in an active form above ground and flowed into a well bore or through the well bore into the subterranean formation.
For example, a subterranean formation is often subjected to a fracturing treatment to stimulate the recovery of fluids, such as crude oil, from the formation. In fracturing, a fluid is introduced in the well bore at a rate and pressure sufficient to produce one or more fractures in the formation and in some instances enlarge or extend existing fractures in a subterranean formation. The fluid can contain a propping agent such as sand or any of the higher strength materials such as resin coated sand, sintered bauxite or ceramic materials which is deposited in the fractures to maintain the fractures open to provide a passageway for the flow of fluids from the formation.
Hydraulic fracturing typically has been performed utilizing a high viscosity gelled aqueous fluid, a high viscosity hydrocarbon/water emulsion or a foam of a gas/water dispersion wherein the gas may initially be admixed either in gaseous or liquid form and subsequently gasified in the formation. These high viscosity fluids are capable of the necessary penetration into the subterranean formation to realize maximum benefits from the fracturing operation and in suspending the propping agent, if present, without excessive settling.
In some instances, viscous fluids described as diverting materials may be introduced into a well prior to or during a fracturing, acidizing, or gravel packing treatments. The viscous fluid which generally is a viscosified aqueous fluid, resists flow into the formation and may be used to obtain multiple fractures in a formation, help prevent undesirable fluids from contacting the formation, divert less viscous fluids and assist in obtaining packer seal in a well bore.
After the high viscosity aqueous fluid or stimulation fluid has been pumped into the subterranean formation and the stimulation treatment performed, it is generally desirable to convert the gel into a low viscosity fluid, thereby allowing the viscous fluid to be removed from the formation and desired material such as gas or oil to flow through the fractures into the well bore for recovery. This reduction in the viscosity of the viscosified fluid often is referred to as "breaking" the gel. Conventionally this breaking is effected by adding a viscosity reducing agent, commonly referred to as a "breaker," to the formation at the desired time. Unfortunately, the conventional known techniques often result in insufficient breaking when breaker concentrations are low, that is, insufficient reduction in the viscosity of the fluid, or premature breaking of the fluid when breaker concentrations are too high. Premature breaking of the fluid causes a reduction in the viscosity of the fracturing fluid or diverter prior to the desired termination of the stimulation operation, thereby reducing the overall effectiveness of the operation.
One method which has been proposed for controlling or delaying the activity of a breaker has been introduction of the active breaker material in a hollow or porous crushable bead or in encapsulated pills or pellets. The viscosity reducing agent or breaker is released immediately upon crushing of the beads which generally results from closing of the formation upon termination of the pumping operation. In some instances premature release can occur if the beads are broken during passage through the high pressure pumping equipment at the well bore. Inconsistent performance also can result in the formation because the stresses caused by the closing of the formation may be such that a large percentage of the beads remain unbroken, particularly if the closure is at a slow rate or a large number of beads may be crushed in one area while another has a much smaller quantity crushed resulting in an insufficient break in the fluid. The pills or pellets typically release the breaker upon hydration of the encapsulating agent which can vary over significant time periods.
Another method has utilized a coated breaker in which the coating is both crushable and permeable to at least one fluid in the formation whereby breaking can be effected by either mode of release.
Although the foregoing methods appear to provide for delayed release of a breaker into a subterranean formation, it remains desirable to provide an alternative method which is capable of providing equivalent or superior performance in commercial applications.